The present invention relates to a toner level sensor for detecting the presence or absence or the level of residual amount of a toner for an electronic copier or the like, or more in particular to a toner level sensor which operates stably regardless of changes of external environmental conditions such as temperature or humidity.
In conventional toner level sensors, as shown in FIG. 1, a transformer 8 including a primary coil 2 and a secondary coil 3 wound on a magnetic core 1 having a magnetic gap 20 is used, so that the output of the secondary coil 3 is positively fed back through an amplifier 4 thereby to form an oscillation loop. When a toner 5 having magnetism is located in the vicinity of the magnetic gap of the magnetic core 1, the coupling coefficient of the magnetic circuit changes with the level of residual amount of the toner, with the result that the feedback rate .beta. changes, and therefore the oscillation level changes as shown in FIG. 2. Thus, by adjusting and setting appropriately the coupling coefficient of the transformer 8 by a fine adjustment system (not shown), it is possible to identify and detect the level B with the residual amount of toner or the level A without any residual amount of toner.
In the above-mentioned conventional toner level sensor shown in FIG. 1, however, the oscillation level should ideally change stepwise with .mu..beta.=1 as a boundary where .beta. is the amount of feedback and .mu. the amplification factor of an amplifier of the oscillation circuit. Actually, however, as shown by a solid line 6 in FIG. 2, the oscillation level rises gently and approaches a maximum value through an intermediate rise state. The intermediate state of this oscillation level is very sensitive to the external conditions such as temperature or humidity, and therefore a drift D is often caused as shown by a dashed line 6a and a dashed line 6b in FIG. 2. As a result, in the case where the detection of the toner level is set as A and B in FIG. 2 as mentioned above, such a disadvantage occurs that it may be utterly impossible to detect the toner level due to the change of feedback amount caused by the drift.
This effect of drift may be avoided by adding a temperature compensating circuit, for instance, in which case the problem is an increased number of component parts. Another problem point is that since the causes of the change of the oscillation level at the intermediate state are complicated, full compensation therefor is very difficult in view of the product variations.